In general, water suppliers take water from water sources such as rivers and treat the taken water in water purification facilities to a drinking water level. The water treated to the drinking water level is supplied to customers as tap water. The customers use water, for example, in a bath, a washroom, and a kitchen and for a washing machine and a toilet, and discharge the used water as waste water. The discharged waste water is treated in sewage treatment facilities to a given level of water quality. The treated waste water is then discharged to rivers.
Water sources are finite and may fail to secure a sufficient water supply during drought. Furthermore, tap water used by the customers is discharged as waste water, and all of the waste water is treated in the sewage treatment facilities. This disadvantageously increases sewage treatment costs.